This invention relates to the alkylation of resorcinol to produce mono-substituted alkylresorcinol isomers. More particularly, this invention relates to the alkylation of resorcinol to produce predominantly 5-alkylresorcinol along with 2-alkylresorcinol.
Phenolic compounds and especially phenol have been alkylated by many methods in the art. Phenols have been alkylated both randomly and selectively by reaction with an olefin, alcohol or ether in the presence of a particular catalyst. One method of preparing ortho substituted alkylphenols is by the vapor phase reaction of at least one alcohol with at least one phenol in the presence of at least one titanium dioxide catalyst. One method for randomly alkylating a phenolic compound is by reacting the phenolic compound with an alcohol or an ether in the vapor phase in the presence of a metal oxide condensation catalyst at a superatmospheric pressure. In this latter method, the phenolic compound is usually a phenol but it may also be a polyhydric phenol like resorcinol. When resorcinol is used, it is converted into monoethyl-, diethyl-, triethyl-, or tetraethyl resorcinol. Neither of these methods are used to alkylate resorcinol partially, i.e., to limit alkylation to one alkyl group per aromatic ring, while alkylating resorcinol selectively, i.e., alkylation in a particular position on the aromtic ring.
Resorcinol is a polyhydric phenol with a hydroxyl group at the 1 and 3 positions on the aromatic ring. Chemical reactions of resorcinol involve both the two hydroxyl groups and the activated nuclear hydrogens in a manner which resembles phenol but is in many respects peculiar to resorcinol. Resorcinol is highly reactive because of the reinforcing action of the two hydroxyl groups. Resorcinol is substituted initially at the position on the aromatic ring where the two hydroxyl groups reinforce thus yielding a resorcinol substituted in the fourth position on the aromatic ring. Any selective partial alkylation of resorcinol at a position other than the fourth position presents inevitable difficulties.
The synthetic routes to an alkylresorcinol that is partially alkylated at a position other than the 4th position on the aromatic ring are multistep and give low overall yields. For example, a 50% yield of a 2-methylresorcinol is obtained by hydrogenation of resorcinol and methylation of the resulting dihydroresorcinol to 2-methylcyclohexane-1,3-dione which on treatment with bromine is converted into 4,6-dibromo-2-methylresorcinol and finally hydrogenolysis of the dibromo derivative to produce the 2-methylresorcinol. Another example is that 5-methylresorcinol can be prepared by a five-step synthesis starting with p-toluidine or a four-step synthesis starting from ethyl crotonate. The five-step synthesis involves acetylation, two-step nitration, reductive hydrogenation, and hydrolysis.
Illustrative of the prior art pertinent to alkylation of phenolic compounds with alcohols are U.S. Pat. Nos. 2,448,942 (Winkler et al.); 3,642,912 (Sharp et. al.); 2,678,951 (Smith et al.); 3,422,156 (Thoma); and 3,426,358 (Schlechting et al.). The process of the present invention is deemed patentable over this prior art because this art fails to teach or disclose a process for producing mono-substituted alkylresorcinol isomers, predominantly the 5-alkylresorcinol isomer along with smaller amounts of 2-alkylresorcinol isomer and 4-alkylresorcinol isomer.
The alkylresorcinol isomers produced by the alkylation of resorcinol with an aliphatic alcohol are useful as chemical intermediates, corrosion inhibitors, photographic chemicals and as components of epoxy resins and polymers.
It is the primary object of this invention to provide a process to produce the three alkylresorcinol isomers, 2-, 4-, and 5-alkylresorcinol, simultaneously in one step.
It is a further object of this invention to provide a process to produce the three alkylresorcinol isomers in such a manner that the major alkylresorcinol isomer produced is the 5-alkylresorcinol along with 2-alkylresorcinol and both can be separated in good yields.